But he has changed his mind, in part because of the time he, too, has spent among the Aka. There are differences, he says, but they are not crucially important, and are often a reflection of who is the primary caregiver. The more involved a man is, Dr. Lamb says, the more he is the hands-on, responsible parent whose job it is to take care of the child.

Mr. Charron, for example, is the primary caregiver for Sophie, now that his wife, Pascale Groulx, has returned to work. He feeds her, changes her diapers, wipes her nose and teaches her not to eat sand. At night, when she is sick or upset, she often wants him.

Dr. Lamb says more fathers are taking on the minute-by-minute jobs of parenting young children.

"I started studying American dads in the mid-seventies, and quite a few men had never changed a nappy," he says. " Many others had done so, but only under duress or extreme conditions.

"You don't find men like that any more."

Spend a few hours in a local park and you see what he means. Fathers are wiping noses, changing diapers and teaching their kids to take turns on the slide.

In the 1970s, a Canadian father's time with children averaged about 40 per cent of the time a mother spent. By the late 1990s, it had jumped to 67 per cent, and in 2004, 9.5 per cent of fathers claimed or planned to claim employment insurance while they took time off to be with their new baby.

But they still like to fling their children up in the air or dangle them by their feet. "I toss her around a little bit more," Mr. Charron says of Sophie. And when the youngsters are a bit older, their dads will begin to play proper games with them, often focusing on sports they loved while they were growing up.

Yet, Dr. Lamb says this physical kind of play is not a universal phenomenon. You see it more in North America, Britain and Australia, he says, but not in the Middle East, Sweden and some countries in Europe.

Daniel Paquette, meanwhile, argues that mother and fathers still have distinct biological roles.

According to the researcher at the University of Montreal, mothers typically comfort children, and calm them down during times of stress. Fathers open them to world, make them braver, and help them compete.

Look at how fathers typically hold babies, he says -- against their shoulders, with their little faces looking out at the world.

The researcher is now working on experiments to test his theory, and he says that a small study in France found that children do better when their parents play different roles.

Dr. Lamb isn't so sure. "We want to be really careful about implying hard-wired differences between men and women," he says.

He says there are some things only a mother can do, like lactation. Fathers may try to let their babies suckle, but even if their breasts produce something, it wouldn't be nutritionally valuable.

Dr. Lamb doubts that males ever breastfed for real, even early in human history, because it takes so much energy to produce milk.

Nipples are just something that comes in handy for Aka dads when they have a cranky baby on their hands.

But men in the West can keep their shirts on and still have a happy baby, Dr. Lamb says. "We have pacifiers."

Anne McIlroy is The Globe and Mail's science reporter.

Superdads

of the animal world

Dayak fruit bats of Malaysia:

Researchers were shocked to discover that the males of this species have mammary glands that appear to produce milk. They don't know if the bats (the first case of wild male mammals producing milk) actually feed their young.

Common marmosets and

cotton-top tamarins: On average, these tiny primates gain 10 per cent of their body weight when their partners are pregnant -- energy they will need once the babies are born. The monkeys usually have twins, relatively large infants about 20 per cent of an adult's size. It is mostly fathers who carry them around on their backs.

Djungarian dwarf hamsters:

The males of this Siberian species act as midwives, delivering their own infants using their mouths and four paws. They lick the pup's nose to clear the airways, then eat the placenta, clear off the membranes and tuck the infant hamster into bed.

Murres: These seabird dads jump off of cliffs with their chicks to teach them to swim.

Seahorses: Males brood their babies in a pouch, which is nourished with a placenta-like mechanism.

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